Should have stressed the "over time" part more. Yes PSA is sensitive to sex and sport (esp. cycling), and also to inflammation. Plus you personal PSA value depends on the actual size of your prostate.
That's why getting an early PSA baseline is important, so you know what value is "good". Also you might not want to take drastic action based on a single "bad" PSA.
No, you got it wrong. You get a baseline PSA measurement AFTER the diagnosis. That way you can see if the patient responds to treatment and also monitor for recurrence.
Cancer antigens are NEVER to be used for diagnosis. Chemotherapy is too destructive to be used without 100% certainty. We've had a woman with high CA125 and it turns out it was tuberculosis in the reproductive system... Everyone wanted to start chemo, the symptoms matched etc...
PSA is definitivly used for early screening prior to diagnosis [1].
But we might use the word baseline for different things.
As PSA is influenced by the patients physiology and activity [1], it can be helpful to understand what the non-cancer baseline/standard PSA value for a specific patient is, this would need to be established by multiple tests over a couple of years. This can help in distinguishing noise from signal later.
After surgery the PSA baseline is zero (by definition, the generating organ has been removed). Any value above zero indicates either metastasis in other body parts or a really bad surgeon.
After non-surgery primary treatment (radiation, ...) the PSA does not (necessarily) fall to zero, so here one takes the lowest value after treatment (with a grace period) as the baseline.
In my father's case, our family doctor requested a prostate cancer screening after seeing PSA levels from regular blood tests rise over the course of a year. We received the cancer diagnosis after the screening.
That's why getting an early PSA baseline is important, so you know what value is "good". Also you might not want to take drastic action based on a single "bad" PSA.